The present invention relates to an improved method for carrying out a Rankine cycle, and more especially to such a method for recovering waste heat by using a fluoro-chloro-hydrocarbon working medium.
Efforts to exploit new sources of energy are directed, inter alia, to the recovery and utilization of existing waste energies through the use of advanced technologies. Frequently, however, the energy involved is in the form of low temperature heat within the range, for example, of 20.degree. C. to 300.degree. C., which is still unused and is being discharged in large amounts into the environment. These waste heat fluxes include the waste gases of industrial conversion processes and streams of cooling water. Conversion of such low grade waste heats into electric energy has not been economically feasible heretofore, with the known technologies.
In conventional thermal power stations, the conversion of heat to electricity using water as the working medium takes place with the water in the liquid form and as steam. The use of water for this purpose is, however, advantageous only when the available heat is at a temperature above approximately 250.degree. C. At temperatures between, for example, 80.degree. and 250.degree. C. water (steam) is not a suitable working medium, because, by virtue of its high specific volume it leads to relatively large and expensive apparatus and machinery.
Theoretical considerations and calculations generally lead to the recognition that, in particular, organic liquids with boiling temperatures significantly lower than that of water and with low specific volumes are especially suitable for this field of application, if, in addition to the above-mentioned requirement, they satisfy the following criteria:
They are non-combustible, non-toxic, thermally stable, chemically inert and low in cost.
Numerous fluoro-chloro-hydrocarbons satisfying most of the above-mentioned requirements are suitable as working and circulating media. Designers will select from the available number of fluoro-chloro-hydrocarbons those offering the best characteristics for the temperature range envisioned, such as, for example, CFCl.sub.3 (R11), CF.sub.2 Cl.sub.2 (R12), CHF.sub.2 Cl (R22), CF.sub.2 Cl--CFCl.sub.2 (R113), CF.sub.2 Cl--CF.sub.2 Cl (R114), CF.sub.3 -CF.sub.2 Cl (R115) or the like. These fluoro-chloro-hydrocarbons have demonstrated their suitability for technical use as cooling media in refrigerating assemblies for a long period of time and in large volumes.
As with other working media, for example, water, the use of fluoro-chloro-hydrocarbons as the working medium in Rankine processes (expansion processes) with respect to the yield in energy becomes more favorable with a rising difference in temperature between the evaporation and the condensation temperature. However, the fluoro-chloro-hydrocarbons which are preferably used, i.e., R 11, R 12, R 22, R 113, R 114 and R 115, tend to more or less strongly decompose with rising temperature within a temperature range of 80.degree.-250.degree. C. in the presence of steel and/or steel alloys, such as those used in the construction of power stations.
The following data is found in this context, in the Matheson Gas Data Book:
rate of decomposition of R 11 at 200.degree. C. in steel: 2%/year PA1 rate of decomposition of R 113 at 200.degree. C. in steel: 6%/year
In "Technical Bulletin B2" (1957)--Freon.sup.R (DuPont), in Table 1--Thermal Stability of Freon.sup.R Compounds--the following limiting temperatures of thermal stability are given:
______________________________________ Maximum Temperature at Constant Exposure Compound in the Presence of Oil/Steel and Copper (.degree.C.) ______________________________________ R 11 107 R 113 107 R 12 121 R 114 121 R 22 135-149 R 13 149 ______________________________________
These data refer generally to "static" experiments, i.e., the fluoro-chloro-hydrocarbon and the metal specimen are exposed in a closed vessel to different test temperatures.
The material used is of decisive importance for the thermal stability of fluoro-chloro-hydrocarons. In the Matheson Gas Data Book, for the compounds R 11, R 12, R 22, R 113, and R 114, at elevated temperatures the following order of metal, in descending order of decomposition, is given: (max. decomposition) silver&gt;brass&gt;bronze&gt;aluminum&gt;1340 steel&gt;copper&gt;nickel&gt;18-8 steel&gt;Inconel.sup.R (min. decomposition).
For the technical application of fluoro-chloro-hydrocarbons in Rankine processes, this signifies that extended and problem-free operation in thermal power stations may be achieved only if
either the working temperatures in such systems--when a carbon steel is used (for example St 39)--are kept very low, which would extensively restrict the economy of such an installation,
or expensive steel alloys and/or special materials, for example, nickel, are employed. In this case again, the economy is strictly limited as a result of the high investment cost.
It is of decisive importance that such "waste heat recovery installations" be able to operate for long periods of time without failures and as free of maintenance as possible. This is achievable only if the thermolysis of the fluoro-chloro-hydrocarbon working medium is prevented or if the thermolysis products are removed from circulation. The term "thermolysis" is intended to signify that the working medium is thermally decomposed at elevated temperatures, whereby firstly the acid dissociation products of chloride and fluoride (in the form of hydrogen chloride and hydrogen fluoride, and possibly also chlorine and fluorine) are formed, and secondly, numerous organic halide by-products are generated.
The hydrolysis of fluoro-chloro-hydrocarbons caused by the residual water content in the working medium, which leads to HCl and HF in highly thermally stressed fluoro-chloro-hydrocarbon circulation systems, is particularly critical for the process. These hydrogen halide acids cause substantial corrosion. Furthermore, these iron-halogen compounds, such as, for example, FeF.sub.3 and FeCl.sub.3 may act as catalysts for isomerization and disproportionation reactions with the fluoro-chloro-hydrocarbons, potentially leading to a change in the thermodynamic and chemical behavior of the fluoro-chloro-hydrocarbon employed.